5.4.1 Nutrient Cycles

The Nitrogen Cycle

Nitrogen is essential for biological molecules such as proteins and nucleic acids. Although nitrogen gas (N₂) makes up ~78% of the atmosphere, it is inert and cannot be used directly by most organisms. The nitrogen cycle ensures nitrogen is recycled and made biologically available.

1. Nitrogen Fixation:

• Nitrogen-fixing bacteria in the soil (e.g., Rhizobium) or root nodules of legumes convert nitrogen gas into ammonia (NH₃), which dissolves in water to form ammonium ions (NH₄⁺).
• This process can also occur abiotically through lightning or the Haber process.

2. Ammonification:

• Saprobionts (decomposers like fungi and bacteria) break down organic matter from dead organisms and waste products (e.g., urea) into ammonium ions.

3. Nitrification:

• Nitrifying bacteria convert ammonium ions into nitrites (NO₂⁻) and then into nitrates (NO₃⁻), which plants can absorb.
• Nitrosomonas converts ammonium ions into nitrites.
• Nitrobacter converts nitrites into nitrates.

4. Assimilation:

• Plants absorb nitrates from the soil and use them to synthesise biological molecules such as amino acids.

5. Denitrification:

• In anaerobic conditions (e.g., waterlogged soils), denitrifying bacteria convert nitrates back into nitrogen gas (N₂), reducing soil fertility.

The Phosphorus Cycle

Phosphorus is an essential component of molecules like ATP, DNA, and phospholipids. Unlike nitrogen, phosphorus does not have a gaseous phase and relies on the movement between land, water, and organisms.

6. Weathering of Rocks:

• Phosphate ions (PO₄³⁻) are released from rocks through weathering and enter soils and water bodies.

7. Absorption by Plants:

• Plants absorb phosphate ions from the soil and incorporate them into biological molecules.

8. Feeding and Digestion:

• Animals obtain phosphorus by consuming plants or other animals and use it to synthesise molecules like ATP and nucleic acids.

9. Decomposition:

• When plants and animals die, saprobionts break down organic matter, returning phosphate ions to the soil or water.

10. Sedimentation:

• Phosphate ions in water bodies can form sediments and eventually reform rocks, completing the cycle.

Human Impacts on Nutrient Cycles:

• Fertiliser Use: Excess nitrates and phosphates from fertilisers can leach into water bodies, causing eutrophication.
• Deforestation: Reduces plant uptake of nutrients, leading to nutrient loss from soil.
• Animal Farming: Produces waste rich in ammonium, altering the nitrogen cycle.